High Zs on C type RCBO Circuits

[crystal ball]

A friend of the family runs a hotel, just had an EICR and the NICEIC electrician has failed some circuits with a marginally high Zs, I seem to recall NICEIC allowing higher readings on RCBO protected circuits, AFAIK the installation is about five years old and is TNCS

 

[JohnW2]

... failed some circuits with a marginally high Zs, I seem to recall NICEIC allowing higher readings on RCBO protected circuits...

I haven't heard of that (which proved nothing), but the moment one starts moving down the road of saying that a Zs too high (even if only 'marginally' so) for the circuit's over-current protection to provide the required disconnection times is OK if the circuit has residual-current protection, one is at risk of sliding all the way to saying that a Zs which is extremely (not just 'marginally') high is OK if one is relying on residual-current devices to provide the fault protection.

Is there a good reason why the devices are Type C?

Kind Regards, John

 

[crystal ball]

Not sure why they are type C, can't see any motors being used in that situation, I was wondering about TT installations too, 1667 ohms max

 

[JohnW2]

Not sure why they are type C, can't see any motors being used in that situation

If there isn't a good reason for them needing to be Type C, that obviously offers a solution (even if potentially a fairly costly one) to resolve the 'EICR fail' problem.

.... I was wondering about TT installations too, 1667 ohms max

I get a bit confused by these numbers - 1,667Ω is the maximum Zs required to cause a 30mA RCD to operate when there is a 50V 'touch voltage' (e.g. between exposed--c-ps and true earth in a TT installation). However, the maximum Zs which would result in the required disconnection times by a 30mA RCD in response to a negligible-impedance L-E fault (which is analogous to the requirement with RMCB/fuse fault protection) would traditionally have been 7,667Ω (at 230V), presumably now 7,283Ω (with 'Cmin').

Kind Regards, John
Edit: typo corrected

 

[ban-all-sheds]

With the disclaimer that this comment is from a few years ago, and terminology may have changed:

In TN systems it is preferable for RCBOs to operate in overcurrent mode when providing indirect shock protection. When they are working in overcurrent mode they are voltage independent, whereas in RCD mode they are not; they need a large enough voltage being applied to drive the circuitry inside. The IEC stipulate that voltage dependent RCDs should operate at voltages as low as 50V, so that the effects of a collapsing voltage in a fault condition are nullified. But if you get an open-circuit neutral, it is possible that an RCBO will not operate in RCD mode with a fault to earth, so the design for a TN system is supposed to ensure that RCBOs operate in overcurrent mode for indirect shock protection, and for this reason the earth fault loop impedance restrictions are the same, and a Type C or D RCBO may not provide a 0.4s disconnection time.